Helical antennas are well known in the art. A standard helix antenna, as described in Kraus, J. D., Antennas (McGraw-Hill 1988), ch. 7, consists of a single wire helically wound in fractions of a turn or in one or more turns along the antenna axis. The helical antenna emits a circularly polarized radio wave that is particularly favored for satellite communication systems owing to the variations in geometry between the received and transmitted signals characteristic of such systems. Electrically short helices (where the number of turns is less than one) produce a nearly circular radiation pattern. It has been found that in order to produce a well defined (directive) beam with maximum gain in the direction of the helical axis, a helical antenna with multiple turns is required. In general, the more turns of the helix the more directive the antenna becomes.
A variation on the single conductor helical antenna is described by C. C. Kilgus in "Resonant Quadrifilar Helix Design," Microwave Journal, (December 1970), pp. 49-54. Kilgus describes a quadrifilar helix antenna having four conductors each .lambda./2 long formed in a 1/2-turn helix. Each of the four legs of this antenna is fed a signal 90.degree. apart in phase (i.e., in phase quadrature). The 1/2-turn quadrifilar helix described by Kilgus produces a hemispherical cardioid (heart-shaped) radiation pattern. The quadrifilar helix structure is preferred for applications requiring a broadbeamed radiation pattern because of the relatively short conductor lengths (.lambda./2) required. Moreover, the quadrifilar helix antenna emits a highly circularly polarized waveform (axial ratio close to 1) over most of its wave pattern. Circular polarization is favored for orbital satellite communication systems owing to the constantly varying geometry between the orbiting satellite and the ground station.
As is the case for the single conductor helix, by increasing the number of turns the quadrifilar helix becomes more directive. In general, multi-turn helical antennas produce a wave pattern with maximum gain in the direction of the axis of the helix (i.e., on boresight). This gain pattern is desirable for many applications, but is generally undesirable for satellites in low Earth orbits. At any point in time, an orbital communication satellite must transmit to users on the surface of the Earth directly along the axis of the helical antenna (i.e., at nadir), as well as at the limb of the Earth's surface (i.e., the communication horizon of the antenna at oblique angles to the helical axis). Because the slant range is far greater for communications at the limb of the Earth than those at nadir for low altitude satellites, communication path losses are greater. Therefore, for these satellites, a more optimal antenna wave pattern would have maximum gain somewhat off the helical axis, with the gain decreased 0-20 dB on boresight.
For the quadrifilar helix antenna, the maximum gain can be shifted away from the main axis by increasing the number of turns and the pitch angle of the helical elements. Therefore, quadrifilar helices of two to five turns are common in low-altitude spacecraft designs, because the radiation pattern can be adjusted to almost perfectly offset the communication path loss. However, the physical dimensions of such antenna systems become quite large. For example, a two-turn antenna operating at a frequency range of 137 to 150 Mhz is up to eight inches in diameter and between 110 and 300 inches long. Moreover, the antenna dimensions must be even larger if more gain is required. In general, the maximum gain that can be achieved by the antenna is proportional to the square root of the helix length. An antenna system of such large dimensions is particularly problematic in spacecraft applications where payload space is at a premium.
One solution to this problem is to create an array of smaller helical antennas having the characteristics of a single large antenna. Arrays of individual antenna elements are common, and the electrical properties of the combination are well known. The gain pattern of an antenna array is defined by the number of array elements, their physical spacing and their electrical phasing (equivalent to their electrical spacing). A desired gain pattern can be achieved with a variety of different array configurations.
Generally, antenna elements can be arrayed axially (longitudinally) or broadside (side-by-side). In prior helical antenna systems, the helical antenna elements have been arrayed broadside separated by distances measured in fractions of or multiples of one or more wavelengths (.lambda.). However, the physical space and side-by-side orientation required by broadside arrays make their implementation less practical for certain applications, such as for space-deployed antenna systems. Moreover, with broadside arrays, independent non-arrayed antennas cannot be interleaved between the arrayed elements because the radiation patterns of the individual elements would interact.
Therefore, there is a need to provide an antenna array comprised of helical antenna elements that is size efficient and that permits independent, interleaved antenna systems or other equipment or payload to be deployed within the physical space between the array elements.